EP3699633A1 - Radar à ouverture synthétique (sar) à bande divisée multi-canaux (mcss) - Google Patents

Radar à ouverture synthétique (sar) à bande divisée multi-canaux (mcss) Download PDF

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Publication number
EP3699633A1
EP3699633A1 EP20152595.3A EP20152595A EP3699633A1 EP 3699633 A1 EP3699633 A1 EP 3699633A1 EP 20152595 A EP20152595 A EP 20152595A EP 3699633 A1 EP3699633 A1 EP 3699633A1
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EP
European Patent Office
Prior art keywords
concurrent
radar pulses
chirps
returns
sar
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EP20152595.3A
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German (de)
English (en)
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Robert M. Taylor
Timothy Earl Durham
Kerry Timothy Speed
Donald A. Lieb
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Eagle Technology LLC
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Eagle Technology LLC
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    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
    • G01S13/00—Systems using the reflection or reradiation of radio waves, e.g. radar systems; Analogous systems using reflection or reradiation of waves whose nature or wavelength is irrelevant or unspecified
    • G01S13/88—Radar or analogous systems specially adapted for specific applications
    • G01S13/89—Radar or analogous systems specially adapted for specific applications for mapping or imaging
    • G01S13/90—Radar or analogous systems specially adapted for specific applications for mapping or imaging using synthetic aperture techniques, e.g. synthetic aperture radar [SAR] techniques
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
    • G01S13/00—Systems using the reflection or reradiation of radio waves, e.g. radar systems; Analogous systems using reflection or reradiation of waves whose nature or wavelength is irrelevant or unspecified
    • G01S13/02—Systems using reflection of radio waves, e.g. primary radar systems; Analogous systems
    • G01S13/06—Systems determining position data of a target
    • G01S13/08—Systems for measuring distance only
    • G01S13/10—Systems for measuring distance only using transmission of interrupted, pulse modulated waves
    • G01S13/22—Systems for measuring distance only using transmission of interrupted, pulse modulated waves using irregular pulse repetition frequency
    • G01S13/227—Systems for measuring distance only using transmission of interrupted, pulse modulated waves using irregular pulse repetition frequency with repetitive trains of uniform pulse sequences, each sequence having a different pulse repetition frequency
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
    • G01S13/00—Systems using the reflection or reradiation of radio waves, e.g. radar systems; Analogous systems using reflection or reradiation of waves whose nature or wavelength is irrelevant or unspecified
    • G01S13/02—Systems using reflection of radio waves, e.g. primary radar systems; Analogous systems
    • G01S13/06—Systems determining position data of a target
    • G01S13/08—Systems for measuring distance only
    • G01S13/10—Systems for measuring distance only using transmission of interrupted, pulse modulated waves
    • G01S13/26—Systems for measuring distance only using transmission of interrupted, pulse modulated waves wherein the transmitted pulses use a frequency- or phase-modulated carrier wave
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
    • G01S13/00—Systems using the reflection or reradiation of radio waves, e.g. radar systems; Analogous systems using reflection or reradiation of waves whose nature or wavelength is irrelevant or unspecified
    • G01S13/88—Radar or analogous systems specially adapted for specific applications
    • G01S13/89—Radar or analogous systems specially adapted for specific applications for mapping or imaging
    • G01S13/90—Radar or analogous systems specially adapted for specific applications for mapping or imaging using synthetic aperture techniques, e.g. synthetic aperture radar [SAR] techniques
    • G01S13/904—SAR modes
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
    • G01S13/00—Systems using the reflection or reradiation of radio waves, e.g. radar systems; Analogous systems using reflection or reradiation of waves whose nature or wavelength is irrelevant or unspecified
    • G01S13/88—Radar or analogous systems specially adapted for specific applications
    • G01S13/89—Radar or analogous systems specially adapted for specific applications for mapping or imaging
    • G01S13/90—Radar or analogous systems specially adapted for specific applications for mapping or imaging using synthetic aperture techniques, e.g. synthetic aperture radar [SAR] techniques
    • G01S13/904—SAR modes
    • G01S13/9056—Scan SAR mode
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
    • G01S13/00—Systems using the reflection or reradiation of radio waves, e.g. radar systems; Analogous systems using reflection or reradiation of waves whose nature or wavelength is irrelevant or unspecified
    • G01S13/88—Radar or analogous systems specially adapted for specific applications
    • G01S13/89—Radar or analogous systems specially adapted for specific applications for mapping or imaging
    • G01S13/90—Radar or analogous systems specially adapted for specific applications for mapping or imaging using synthetic aperture techniques, e.g. synthetic aperture radar [SAR] techniques
    • G01S13/904—SAR modes
    • G01S13/9076—Polarimetric features in SAR
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
    • G01S7/00—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00
    • G01S7/02—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00 of systems according to group G01S13/00
    • G01S7/024—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00 of systems according to group G01S13/00 using polarisation effects
    • G01S7/025—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00 of systems according to group G01S13/00 using polarisation effects involving the transmission of linearly polarised waves
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
    • G01S7/00—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00
    • G01S7/02—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00 of systems according to group G01S13/00
    • G01S7/03—Details of HF subsystems specially adapted therefor, e.g. common to transmitter and receiver
    • G01S7/034—Duplexers
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
    • G01S7/00—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00
    • G01S7/02—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00 of systems according to group G01S13/00
    • G01S7/28—Details of pulse systems
    • G01S7/282—Transmitters
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
    • G01S7/00—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00
    • G01S7/02—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00 of systems according to group G01S13/00
    • G01S7/28—Details of pulse systems
    • G01S7/285—Receivers
    • G01S7/288—Coherent receivers

Definitions

  • the present disclosure relates to synthetic aperture radar.
  • Synthetic aperture radar is highly effective for remote sensing using active microwave transmissions at a variety of wavelengths between L-band and Ku-band, but achieving a large range swath is difficult.
  • the length of the continuous range swath that can be received is limited by the radar reception time window between consecutive transmitted pulses.
  • the maximum time between transmit pulses required to avoid Doppler ambiguities within the antenna beam width is one half the antenna width divided by the platform velocity.
  • This inherent limit on range swath is a significant issue for smaller antennas, especially when the antenna is not tall enough to form a narrow enough beam to suppress radar returns from ambiguous ranges.
  • phased array antennas are often used to electronically steer their radio frequency (RF) beams on the ground to increase the SAR coverage area. These phased arrays must handle high power and are expensive to manufacture.
  • RF radio frequency
  • a method performed by a synthetic aperture radar (SAR) on a flight vehicle comprises: generating concurrent first radar pulses in respective first frequency channels; transmitting, and receiving returns of, the concurrent first radar pulses by respective first antenna feeds configured to form respective first beams in the respective first frequency channels, the respective first beams directed to respective first subswaths of a swath on the Earth separated one from the next by respective subswath gaps; generating concurrent second radar pulses in respective second frequency channels; transmitting, and receiving returns of, the concurrent second radar pulses by respective second antenna feeds configured to form respective second beams in the respective second frequency channels, the respective second beams directed to respective second subswaths of the swath on the Earth separated one from the next to coincide with the subswath gaps; and performing SAR processing on the returns of the first radar pulses from the first subswaths and the returns of the second radar pulses from the second subswaths to form a SAR image contiguous across the swath.
  • Embodiments presented herein overcome the above-mentioned problems, disadvantages, and challenges associated with conventional SARs, and offer advantages that will become apparent from the ensuing description.
  • FIG. 1 there is an illustration of an example satellite 100 that provides a platform for a multi-channel split swath (MCSS) SAR to illuminate on a surface of the Earth a SAR swath along a ground track as the satellite travels through space in a direction substantially parallel to the ground track.
  • the MCSS SAR platform may be any flight vehicle, including, but not limited to, satellite 100, an airplane, aerostat, drone, and the like.
  • FIG. 1 shows an example antenna assembly 102 of the MCSS SAR used to illuminate/form the swath. Further details of the MCSS SAR are described below in connection with FIGs. 7A , 7B , and 8 .
  • Antenna assembly 102 includes a parabolic reflector 104 (e.g., a 4 meter reflector) and an antenna feed assembly 106 that exchange RF signals with each to form multiple antenna beams to transmit energy to the swath in the form of frequency-separated trains of concurrent radar pulses, and to receive returns/reflections of the concurrent radar pulses from the swath, as will be described in detail below.
  • a parabolic reflector 104 e.g., a 4 meter reflector
  • antenna feed assembly 106 that exchange RF signals with each to form multiple antenna beams to transmit energy to the swath in the form of frequency-separated trains of concurrent radar pulses, and to receive returns/reflections of the concurrent radar
  • the MCSS SAR embodiments presented herein employ radar pulses in the form of frequency chirped pulses (referred to simply as "chirps").
  • a chirp is a signal, e.g., a carrier wave, having a frequency that increases with time over a duration of the chirp (i.e., an up-chirp) or decreases with time over the duration of the chirp (i.e., a down-chirp).
  • other MCSS embodiments may employ radar pulses other than chirps.
  • Such radar pulses may each convey a carrier wave that has a frequency that is either constant, or that varies according to a non-chirp frequency-time function, over a duration of the pulse.
  • Antenna feed assembly 106 includes a set A of separate antenna feeds (AFs) 1A-4A that exchange RF feed energy with reflector 104 in transmit and receive directions to form a set A of concurrent, frequency-separated, respective elevation beams (BMs) 1A-4A (see, e.g., FIG. 3 ).
  • the physical separation of each antenna feed 1A-4A causes the set A elevation beams to be directed to different sub-swaths (SSs) 1A-4A within the full radar swath.
  • SSs sub-swaths
  • Each beam operates as both a transmit beam and a receive beam in a time-multiplexed fashion.
  • Beams 1A-4A transmit/receive respective concurrent chirps in frequency-separated channels or bands centered on unique respective frequencies f1-f4.
  • frequencies fl-f4 may be in C-band (e.g., around 5.4 GHz).
  • Antenna feed assembly 106 also includes a set B of separate antenna feeds 1B-4B that exchange RF feed energy with reflector 104 in the transmit and receive directions to form a set B of concurrent, frequency-separated, respective elevation beams 1B-4B (not shown in FIG. 1 , but shown in FIG. 4 ).
  • the physical separation of each antenna feed 1B-4B causes the elevation beams to be directed to different sub-swaths (SSs) 1B-4B that are interleaved in elevation with SSs 1A-4A and slightly offset in cross-range from SSs 1A-4A.
  • SSs sub-swaths
  • Each of the set B beams operates as both a transmit beam and a receive beam in a time-multiplexed fashion.
  • each antenna feed may be any type of antenna feed, including feed horns, patch arrays, cup-dipoles, or an array of several feed horns.
  • antenna feeds 1A-4A are positioned such that their beams 1A-4A illuminate on the Earth set A subswaths (SSs) 1A-4A, respectively.
  • the term "illuminate” connotes the transfer of RF energy in both transmit and receive directions.
  • Set A subswaths 1A-4A are physically separated one from the next by respective ones of the cross-range subswath gaps.
  • antenna feeds 1B-4B are positioned such that their beams 1B-4B illuminate on the Earth set B subswaths 1B-4B, respectively, physically separated one from the next by respective ones of the subswath gaps.
  • Set B is offset in elevation angle so that the subswaths 1B-4B illuminate the subswath gaps of antenna feeds 1A-4A.
  • set A subswaths 1A-4A and set B subswaths 1B-4B form 8 contiguous subswaths of the swath, which has a total width that is equal to a sum of the widths of all the subswaths.
  • each subswath has a width of many kilometers.
  • the MCSS SAR operates or utilizes antenna assembly 102 in a repetitive or cyclical time-multiplexed manner that includes a first time period and a second time period that is time-offset from the first time period.
  • the "first time period/second time period" cycle repeats.
  • the first and second time periods are consecutive in time.
  • only set A antenna feeds 1A-4A form respective beams 1A-4A to illuminate only subswaths 1A-4A of the swath.
  • antenna feeds 1A-4A transmit concurrent chirps in their respective frequency channels f1-f4.
  • Antenna feeds 1A-4A transmit the concurrent chirps at a first pulse repetition frequency (PRF) that is common across the antenna feeds.
  • each antenna feed e.g., antenna feed 1A, antenna feed 2A, and so on
  • the chirps in the respective train of chirps transmitted by each of antenna feeds 1A-4A are time-aligned with, but separated in frequency from, corresponding chirps in the individual trains of chirps transmitted by the other antenna feeds.
  • the antenna feeds 1A-4A collectively transmit a train of concurrent chirps.
  • each individual train of transmitted chirps includes both horizontally polarized (H) and vertically polarized (V) chirps.
  • the chirps may alternate between H chirps and V chirps.
  • the train of concurrent chirps may include alternating H and V concurrent chirps.
  • antenna feeds 1A-4A time-multiplex or alternate between transmit during a transmit period of the concurrent chirps, and receive (i.e., operated in the receive direction) between the transmit periods. In this way, antenna feeds 1A-4A alternately transmit the concurrent chirps, and then receive returns of the concurrent chirps.
  • the receive channel may be configured to receive both horizontal (H) and vertical (V) chirps in each receive period, therefore allowing the radar to measure all 4 possible combinations or transmit/receive polarizations including H/H, V/V, H/V and V/H from alternating H and V transmit chirps.
  • antenna feeds 1B-4B transmit concurrent chirps in respective frequency channels f1-f4.
  • Antenna feeds 1B-4B transmit the concurrent chirps at a second PRF that is common across the antenna feeds, but different from the first PRF.
  • antenna feeds 1B-4B each transmits a respective train of chirps at the second PRF and in the respective frequency channel.
  • the chirps in the individual train of chirps transmitted by each of antenna feeds 1B-4B are time-aligned with corresponding chirps in the trains of chirps transmitted by the other antenna feeds.
  • antenna feeds 1B-4B collectively transmit concurrent chirps.
  • each individual train of chirps in the second time period may include both H and V chirps.
  • the chirps may alternate between H chirps and V chirps.
  • antenna feeds 1B-4B also receive returns of the transmitted concurrent chirps from illuminated subswaths 1B-4B.
  • antenna feeds 1B-4B time-multiplex or alternate between transmit during a transmit period of the concurrent chirps, and receive between the transmit periods. In this way antenna feeds 1B-4B alternately transmit the concurrent chirps, and then receive returns of the concurrent chirps.
  • the receive channel may be configured to receive both horizontal (H) and vertical (V) in each receive period, therefore allowing the radar to measure all 4 possible combinations or transmit/receive polarizations including H/H, V/V, H/V and V/H from alternating H and V transmit chirps.
  • FIG. 2A there is a plot of time vs. frequency for an arrangement of chirps transmitted by the set A antenna feeds and the set B antenna feeds during first and second time periods P1 and P2, respectively, for airborne applications.
  • a pulse e.g., chirp
  • PRI pulse repetition interval
  • a given pulse e.g., chirp
  • the transmit and receive windows are consecutive in time and are repeated until the desired along track image area is obtained.
  • the PRI is the time interval between the start of one chirp and the next, which is the inverse of the PRF.
  • antenna feeds 1A-4A transmit concurrent sets of chirps CA(1), CA(2), CA(3) and so on, at a first PRF PRF 1 during transmit periods T1, T2, T3, and so on, and receive returns of the chirps during receive periods R1, R2, R3, and so on.
  • Each concurrent set of chirps CA(i) includes time-aligned first, second, third, and fourth chirps in non-overlapping frequency channels centered at frequencies f1, f3, f3, and f4, respectively.
  • the frequency of each chirp increases or ramps-up in frequency over a time period of the chirp.
  • the frequency of each chirp may decrease over the time period of the chirp.
  • time period P1 is chosen such that data from enough transmit (T) and receive (R) periods is taken to form a SAR image in each of the subswaths SS1, SS2, and so on.
  • antenna feeds 1B-4B transmit concurrent sets of chirps CB(1), CB(2), CB(3) and so on, at a second PRF PRF 2 different from PRF 1 during time periods T, and receive returns of the chirps during time periods R.
  • PRF 1 and PRF 2 are 3250.9 Hz and 3265.7 Hz respectively.
  • Each concurrent set of chirps CB(i) includes time-aligned first, second, third, and fourth chirps in non-overlapping frequency channels centered at frequencies f1, f3, f3, and f4, respectively.
  • the frequency of each chirp increases or ramps-up in frequency over a time period of the chirp.
  • the frequency of each chirp may decrease over the time period of the chirp, or may be any other similarly frequency limited waveform.
  • the concurrent chirps can alternate between H concurrent chirps and V concurrent chirps, as shown.
  • the repeating transmit and receive windows are shown as is typical for spaceborne applications.
  • the PRF must be much higher than is required for an airborne radar due to the higher velocity of the measurement platform.
  • the PRI for a space radar is much shorter than the travel time of a pulse from the radar to the ground and back. Consecutive transmit and receive time periods are still used; however a given pulse is received a number of PRIs later than when it was transmitted. This is shown in FIG. 2B where the time delay between when a chirp is transmitted and when it is received for the beam 1A is n times the PRI.
  • the other beams are higher in elevation and therefore take longer to return to the space radar at increasing range.
  • the elevation angle of the other beams is chosen such that the chirp returns to the second beam after an interval of n+1 times the PRI and the third beam returns after an interval of n+2 times the PRI, and so forth.
  • the first time period P1 must be extended by n times the PRF to allow all chirps to be received. This design allows all four channels to be sampled simultaneously with a single receiver, but each receive window records the reflected signal from a different transmit period for each respective channel.
  • a process is followed to set up the PRF, delay interval, and elevation angles that is similar to what may be used for a single swath strip-map SAR system, as would be appreciated by one of ordinary skill in the art having access to the present specification.
  • FIG. 3 there is an illustration to show the multiple beams and multiple subswaths of the MCSS SAR. More specifically, FIG. 3 shows set A antenna feeds 1A-4A illuminating the set A subswaths 1A-4A on the surface of the Earth that have chirp returns within the time period P1 reception periods via set A beams 1A-4A. FIG. 3 shows subswath gaps between the illuminated (and chirp fully received) subswaths.
  • set B antenna feeds 1B-4B illuminating the set B subswaths 1B-4B on the surface of the Earth via set B beams 1B-4B during second time period P2.
  • Each of subswaths 1B-4B fills a respective gap between a respective adjacent pair of subswaths among subswaths 1A-4A.
  • FIG. 5 there is an illustration that superposes both set A antenna feeds 1A-4A illuminating the set A subswaths 1A-4A and set B antenna feeds 1B-4B illuminating the set B subswaths 1B-4B on the surface of the Earth.
  • the illustration shows that the full swath is illuminated when the consecutive time periods P1 and P2 are combined.
  • FIG. 6 there is a plot of swath coverage in terms of increasing range for satellite 100 (vertical direction) and satellite track direction/time (horizontal direction).
  • the swath coverage is shown for 3 pairs of repeating time periods P1 and P2, shown in the horizontal direction.
  • the satellite positions during each PRF set is depicted along the bottom.
  • Each simultaneously collected set of 4 subswaths is centered in the along-track direction on the center of the flight path segment flown by the radar platform during the PRF set.
  • the subswaths are over twice as long as the corresponding flight segment to ensure some overlap in the along-track direction with the next set collected at the same PRF.
  • Each set of 4 subswaths labeled 1A-4A corresponds to periods P1, while 1B-4B correspond to periods P2.
  • switch control signals also referred to as "switch signals”
  • timing signals are not shown in FIGs. 7A and 7B ; however, certain of the switch signals are shown in FIG. 8 .
  • SAR 700 includes set A antenna feeds 1A-4A and set B antenna feeds 1B-4B (reflector 104 is not shown in FIG. 7A ), and a digital controller C coupled to the set A and the set B antenna feeds through respective RF paths.
  • SAR 700 also includes a frequency multiplexer 706A(1) coupled to each of antenna feeds 1A-4A, and a frequency multiplexer 706B(1) coupled to each of antenna feeds 1B-4B.
  • SAR 700 also includes a transmit-only ("TX only”) path 708(1) coupled to output OUT of controller C and T/R switch T/R(1) (the T/R switch is coupled to the controller via the transmit-only path and also to frequency multiplexer 706A(1)), and a receive-only ("RX only) path 710(1) coupled to input IN of the controller and T/R switch T/R(1).
  • T/R switch T/R(1) switches between transmit-only paths 708(1) and receive-only path 710(1), respectively, responsive to respective T/R switch signals (not shown in FIG. 7A ).
  • Transmit-only path 708(1) includes a power transmit amplifier TA(1) coupled between output OUT of controller C and T/R switch T/R(1).
  • Receive-only path 710(1) includes a receive amplifier RA(1) coupled between T/R switch T/R(1) and input IN of controller C.
  • Receive amplifier RA(1) may including multiple receive amplifiers and a receive filter connected in series.
  • SAR 700 also includes a subswath switch SSW(1) that selectively couples T/R switch T/R(1) to frequency multiplexer 706A(1) or 706B(1) through the subswath switch, responsive to a subswath switch signal (not shown in FIG. 7A ).
  • controller C In a transmit direction/mode, in a first time period, controller C generates at output OUT a train of concurrent first chirps having frequencies f1-f4 at a first PRF, as shown in FIGs. 2A and 2B . Transmit only path 708(1) forwards the concurrent first chirps to T/R switch T/R(1), which directs the concurrent first chirps to subswath switch SSW(1) during a pulse/ON period of the concurrent first chirps. Subswath switch SSW(1) directs the concurrent first chirps to frequency multiplexer 706(1) over a single RF path.
  • Set A antenna feeds 1A-4A transmit their respective individual frequency-separated chirps, concurrently.
  • frequency multiplexer 706A(1) receives from antenna feeds 1A-4A individual/separate returns of the concurrent first chirps across the frequencies f1-f4, combines the returns by frequency (i.e., frequency-multiplexes the returns) into frequency-combined returns of the chirps, and directs the frequency-combined returns to subswath switch SSW(1) over the single RF path.
  • Subswath switch SSW(1) directs the frequency-combined returns to T/R switch T/R(1).
  • the T/R switch directs the returns of the concurrent first chirps to receive only path 710(1).
  • Receive only path 710(1) directs the frequency-combined returns to input IN of controller C, which processes the returns.
  • controller C outputs from output OUT a train of concurrent second chirps having frequencies f1-f4 at a second PRF that is different from the first PRF, as shown in FIGs. 2A and 2B .
  • the RF flow and operations described above for the transmit direction and the receive direction during the first time period are repeated during the second time period, except that subswath switch SSW(1) directs the concurrent second chirps and their returns to and from frequency multiplexer 706B(1) (and set B antenna feeds 1B-4B) instead of frequency multiplexer 706A(1) (and set A antenna feeds 1A-4A).
  • SAR 750 employs dual receive-only paths and dual polarization, e.g., horizontal (H) and vertical (V) polarization.
  • SAR 750 includes additional RF elements and capabilities relative to SAR 700.
  • SAR 750 includes an H/V switch "H/V" following transmit power amplifier TA(1) to selectively direct concurrent chirps generated by controller C to either transmit-only path 708(1) or a second transmit-only path 708(2) responsive to an H/V switch signal (not shown in FIG. 7B ).
  • set A frequency multiplexer 706A(1) is coupled to horizontally polarized input/output (I/O) ports of set A antenna feeds 1A-4A
  • set B frequency multiplexer 706B(1) is coupled to horizontally polarized I/O ports (i.e., the H ports) of set B antenna feeds 1B-4B, such that RF injected to/received from the H ports will be transmitted from/received from the antenna feeds as horizontally polarized RF.
  • SAR 750 also includes a second set A frequency multiplexer 706A(2) coupled to vertically polarized input/output (I/O) ports (i.e., the V ports) of set A antenna feeds 1A-14, and a second set B frequency multiplexer 706B(2) coupled to the V ports of set B antenna feeds 1B-4B.
  • SAR 750 also incudes a second subswath switch SSW(2) to selectively direct concurrent chirps from second transmit-only path 708(2) to either second set A frequency multiplexer 706A(2) or second set B frequency multiplexer 706B(2) responsive to a second subswath switch signal (not shown in FIG. 7B ).
  • SAR 750 also includes a second receive-only path 710(2) having a receive amplifier RA(2), and a second T/R switch T/R(2) to selectively direct (i) concurrent chirps from second transmit-only path 708(2) to second subswath switch SSW(2), or (ii) returns of the concurrent chirps from the second subswath switch to second receive-only path 710(2).
  • Switch H/V, T/R(1), T/R(2), SSW(1), and SSW(2) are configured to direct concurrent first chirps in alternating fashion to the H ports of set A antenna feeds 1A-4A frequency multiplexer 706A(1), and then to the V ports of set A antenna feeds 1A-4A through frequency multiplexer 706A(2).
  • set A antenna feeds 1A-4A transmit alternating H and V concurrent first chirps, as shown in FIGs. 2A, 2B , and 9 , for example.
  • switches T/R(1), T/R(2), SSW(1), and SSW(2) are configured to direct returns of the alternating H and V concurrent first chirps from the H and V ports of set A antenna feeds 1A-4A to receive paths 710(1) and 710(2) through frequency multiplexers 706A(1) and 706A(2), which provide the returns to input IN of controller C for processing.
  • the RF flow and operations described above for the transmit direction and the receive direction during the first period are repeated during the second time period, except that subswath switches SSW(1) and SSW(2) direct the alternating H and V concurrent second chirps and their returns to and from frequency multiplexers 706B(1) and 706B(2) (and set B antenna feeds 1B-4B) instead of frequency multiplexers 706A(1) and 706A(2) (and set A antenna feeds 1A-4A).
  • MCSS SAR there are numerous other possible embodiments for the MCSS SAR that range in complexity.
  • another embodiment may employ a dual, fully redundant transmit-only path, dual receive-only paths, and dual polarization.
  • Controller C includes a chirp generator 802 and a return chirp processor 804.
  • Chirp generator 802 includes a baseband chirp generator 806 that generates a train of digitized baseband chirps 808 at a PRF indicated by a PRF control signal.
  • Chirp generator 802 includes parallel digital mixers M1-M4 to frequency-upconvert digitized baseband chirps 808 to separate digitized concurrent RF chirps 810(1)-810(4) centered at RF frequencies fl-f4, respectively.
  • Chirp generator 802 also includes a combiner 812 to combine separate digitized concurrent RF chirps 810(1)-810(4) into a combined digitized RF signal including the concurrent chirps (i.e., a frequency-multiplexed signal), and a digital-to-analog converter (D/A) 814 to convert the combined digitized RF signal to an analog RF signal including the concurrent chirps.
  • D/A 814 feeds the analog RF signal to transmit-only paths 708(1), 708(2).
  • Return chirp processor 804 includes an analog-to-digital converter (A/D) 820 to receive analog RF returns 822 of concurrent chirps from receive-only paths 710(1), 710(2). A/D 820 digitizes the analog RF returns to produce digitized RF returns.
  • Return chirp processor 804 includes a chirp separator and frequency-downconverter 823 to separate the return chirps by frequency, and frequency-downconvert the chirps to baseband, to produce frequency-separated returns chirp signals 824(1)-824(4) corresponding to frequencies f1-f4, respectively, at baseband.
  • Return chirp processor 804 includes a SAR processor 830 that receives mappings between frequencies f1-f4 and subswaths 1A/1B-4A/4B, first and second time period indicators, and frequency-separated chirp signals 824(1)-824(4). SAR processor 830 processes the returns of the concurrent chirps from the 8 subswaths as indicated in chirp signals 824(1)-824(4) in the first and second time periods, to generate a complete SAR image of the swath.
  • any known or hereafter developed SAR processing technique may be used to form a contiguous SAR image of the swath from returns of concurrent chirps from the subswaths (which may include simultaneous V and H returns of alternating V and H chirps), as would be appreciated by one of ordinary skill in the relevant arts.
  • Controller C may also include a switch controller 840 to generate two T/R switch signals 842 to control T/R switches T/R(1), T/R(2), two subswath switch signals 844 to control subswath switches SSW(1), SSW(2) (and the time period indicator signal), and an H/V switch signal 846 to control the H/V switch.
  • switch controller 840 may be separate from controller C.
  • chirp generator 802 and return chirp processor 804 generate and process chirps. It is understood that chirp generator 802 and return chirp processor 804 may also be modified to generate and process pulses other than chirps and that translate to non-chirp radar pulses, for example, pulses that convey carrier waves at a constant frequency, and so on.
  • Controller C may also include one or more processors 860 to execute software stored in a memory 862.
  • Processors 860 may interface with D/A 814 and A/D 820 through an I/O interface 861.
  • Processor(s) 860 may include, for example, one or more microprocessors and/or microcontrollers.
  • the memory 862 stores instructions for software stored in the memory that are executed by processor(s) 860 to perform the operations described herein.
  • Memory 862 may comprise read only memory (ROM), random access memory (RAM), magnetic disk storage media devices, optical storage media devices, flash memory devices, electrical, optical, or other physical/tangible memory storage devices.
  • the memory 862 may comprise one or more tangible (non-transitory) computer readable storage media (e.g., a memory device) encoded with software comprising computer executable instructions and when the software is executed (by the processor(s) 860) it is operable to perform the operations described herein.
  • Memory 862 may store processing logic 864 to implement and/or control baseband chirp generator 802, return chirp processor 804, and switch controller 840.
  • memory 862 stores data 866 used and generated by the processor(s) 860 when executing the logic described above.
  • FIG. 9 there is an illustration of a given set of antenna feeds (e.g., either set A or set B antenna feeds) transmitting alternating H and V concurrent chirps at a given PRF during a time period to illuminate subswaths on the surface of the Earth.
  • alternating H and V concurrent chirps returns from the surface of the Earth are not received/measured/processed due to the shared receive/transmit paths of SAR 700. This results in the gaps between the subswaths, referred to as ground subswaths with transmit pulse blanking.
  • transmitting from a second set of HFs alternating H and V concurrent chirps at a different PRF during another time period fills-in the gaps.
  • FIG. 10 there is a flowchart of an example SAR method 1000 performed by any of the MCSS SARs described above, e.g., MCSS SAR 700.
  • the SAR generates concurrent first radar pulses (e.g., chirps) in respective first frequency channels (e.g., the first radar pulses convey respective carrier waves in, i.e., that coincide with, respective first frequency channels) and at a first PRF.
  • first radar pulses e.g., chirps
  • respective first frequency channels e.g., the first radar pulses convey respective carrier waves in, i.e., that coincide with, respective first frequency channels
  • the SAR alternately transmits, and receives returns of, the concurrent first radar pulses (e.g., chirps) by respective first antenna feeds configured to form respective first beams in the respective first frequency channels, the respective first beams directed to respective first subswaths of a swath on the Earth separated one from the next by respective subswath gaps.
  • the concurrent first radar pulses e.g., chirps
  • respective first antenna feeds configured to form respective first beams in the respective first frequency channels, the respective first beams directed to respective first subswaths of a swath on the Earth separated one from the next by respective subswath gaps.
  • the SAR generates concurrent second radar pulses (e.g., chirps) in respective second frequency channels (e.g., the first radar pulses convey respective carrier waves in/that coincide with respective first frequency channels) and at a second PRF.
  • second radar pulses e.g., chirps
  • respective second frequency channels e.g., the first radar pulses convey respective carrier waves in/that coincide with respective first frequency channels
  • the SAR alternately transmits, and receives returns of, the concurrent second radar pulses (e.g., chirps) by respective second antenna feeds configured to form respective second beams in the respective second frequency channels, the respective second beams directed to respective second subswaths of the swath on the Earth separated one from the next to coincide with the subswath gaps.
  • the concurrent second radar pulses e.g., chirps
  • the SAR performs SAR processing on the returns of the first radar pulses (e.g., chirps) from the first subswaths and the returns of the second radar pulses (e.g., chirps) from the second subswaths (which may both include returns of V and H radar pulses chirps) to form a SAR image contiguous across the swath.
  • first radar pulses e.g., chirps
  • the second radar pulses e.g., chirps

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EP20152595.3A 2019-02-22 2020-01-20 Radar à ouverture synthétique (sar) à bande divisée multi-canaux (mcss) Pending EP3699633A1 (fr)

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